Electrolytic capacitors typically comprise a metal oxide dielectric layer which is electrochemically formed on the surface of a metal anode. Such capacitors may be, for example, as disclosed in U.S. Pat. Nos. 4,609,971 and 5,143,591, both commonly assigned herewith. In such capacitors, for example, a conductive electrolyte is disposed in intimate contact with the outer surface of the dielectric. The metal anode forms one electrode of the capacitor while the conductive electrolyte forms the second electrode. The use of such metal anodes commonly involves anodization. Anodization is a well known electrochemical reaction wherein a film is grown on the anode in an electrochemical cell according to the basic equations EQU M+nH.sub.2 O.fwdarw.MO.sub.n +2nH.sup.+ +2ne (at the anode) EQU 2ne2nH.sub.2 O.fwdarw.nH.sub.2 +2nOH.sup.- (at the cathode)
An oxide is formed on the metal anode surface and hydrogen is evolved at the cathode. Anodization usually is undertaken in an aqueous electrolyte. At the anode, the reaction with the larger electronegative potential will occur. Oxide films can be prepared by methods well known in the art wherein a constant current is passed through the cell, the film thickness is proportional to the time for which the current is passed, and the voltage developed across the oxide film is a measure of the film thickness; or film may be formed under constant voltage conditions, all as is well known in the art.
Various processes are used to etch aluminum foil to make it suitable for use as and to enhance its properties when it is used as the anode in such electrolytic cells as described above. For example, it is customary to etch the foil surfaces to form microscopic holes and thereby increase the effective surface area (which foil surfaces may be masked to etch only selected areas as desired), and then to subject the foil to anodic oxidation to increase the electrostatic capacity of an electrolytic capacitor using such a foil. However, in the conventional etching techniques, dissolution of the aluminum foil surface either tends to proceed preferentially over the growth of the etching holes or etching proceeds slowly and inefficiently because of the ineffectiveness of the etch electrolyte and/or the rapid depletion of one or more essential components of the etch electrolyte.